Optical imaging through turbid media with a degenerate four-wave mixing correlation time gate

Abstract

Optical imaging through turbid media is demonstrated using a degenerate four-wave mixing correlation time gate. An apparatus and method for detecting ballistic and/or snake light while rejecting unwanted diffusive light for imaging structures within highly scattering media are described. Degenerate four-wave mixing (DFWM) of a doubled YAG laser in rhodamine 590 is used to provide an ultrafast correlation time gate to discriminate against light that has undergone multiple scattering and therefore has lost memory of the structures within the scattering medium. Images have been obtained of a test cross-hair pattern through highly turbid suspensions of whole milk in water that are opaque to the naked eye, which demonstrates the utility of DFWM for imaging through turbid media. Use of DFWM as an ultrafast time gate for the detection of ballistic and/or snake light in optical mammography is discussed.

What is claimed is: 
     
       1. An apparatus for imaging structure in a turbid medium, which comprises in combination: a. means for generating first, pulsed electromagnetic radiation having a chosen wavelength and coherence time;   b. means for separating the first, pulsed electromagnetic radiation into a probe beam and a pump beam;   c. an optical cell containing a chosen material such that a portion of the first, pulsed electromagnetic radiation entering said optical cell is absorbed, while the remainder thereof may exit said optical cell;   d. means for directing the pump beam into said optical cell;   e. means for directing the probe beam through the turbid medium, whereby the portion of the probe beam which exits the turbid medium and which contains ballistic and snake components, which maintain information concerning the structure, and a diffusive component which has lost such information, enters said optical cell such that the probe beam and the pump beam may interfere with one another, creating thereby a grating having sinusoidal spatial variation in refractive index at the chosen wavelength of first pulsed electromagnetic radiation in the material;   f. means for introducing an optical delay into the probe beam to compensate for path differences between the probe beam and the pump beam;   g. means for generating a second, pulsed electromagnetic radiation having a chosen wavelength and directing the second, pulsed electromagnetic radiation into said optical cell approximately collinear to, but in the opposite direction to the pump beam, whereby the second, pulsed electromagnetic radiation is scattered by the grating formed in the chosen material in said optical cell; and   h. means for detecting the scattered electromagnetic radiation, whereby an ultrafast time gate is generated in the chosen material in said cell which discriminates between the ballistic and snake components, and the diffusive component of the probe beam which exit the turbid medium.   
     
     
       2. The apparatus for imaging structure in a turbid medium as described in claim 1, wherein said means for generating a second, pulsed electromagnetic radiation having a chosen wavelength and for directing the second, pulsed electromagnetic radiation into said optical cell approximately collinear to, but in the opposite direction to the pump beam, includes means for retroreflecting pump beam radiation unabsorbed by the chosen material in said optical cell back into said optical cell, whereby the resulting scattered electromagnetic radiation is the phase conjugate of the probe beam. 
     
     
       3. The apparatus for imaging structure in a turbid medium as described in claim 2, wherein a chosen delay is introduced in the retroreflected unabsorbed electromagnetic radiation by said retroreflecting means such that the delay between the grating formation and the scattering of the retroreflected unabsorbed electromagnetic radiation is optimized. 
     
     
       4. The apparatus for imaging structure in a turbid medium as described in claim 1, wherein said first, pulsed electromagnetic radiation generating means includes a frequency-doubled Nd:YAG laser, and the chosen material includes a solution of rhodamine 590. 
     
     
       5. The apparatus for imaging structure in a turbid medium as described in claim 1, wherein means are provided for forming the pump beam into a thin sheet which traverses said optical cell substantially along the front surface thereof in order that the probe beam traverse the minimum amount of chosen material before interacting with the pump beam. 
     
     
       6. The apparatus for imaging structure in a turbid medium as described in claim 1, the chosen wavelength of the first, pulsed electromagnetic radiation is selected such that the chosen wavelength is off resonance with the chosen material contained in said optical cell, so that absorption of the pump beam and probe beam in said optical cell are minimized. 
     
     
       7. A method for imaging structure in a turbid medium, which comprises the steps of: a. generating first, pulsed electromagnetic radiation having a chosen wavelength and coherence time;   b. separating the first, pulsed electromagnetic radiation into a probe beam and a pump beam;   c. directing the pump beam into an optical cell containing a chosen material such that a portion of the first, pulsed electromagnetic radiation entering the optical cell is absorbed, while the remainder thereof may exit the optical cell;   e. directing the probe beam through the turbid medium, whereby the portion of the probe beam which exits the turbid medium and which contains ballistic and snake components, which maintain information concerning the structure, and a diffusive component which has lost such information, enters the optical cell such that the probe beam and the pump beam may interfere with one another, creating thereby a grating having sinusoidal spatial variation in refractive index at the chosen wavelength of first pulsed electromagnetic radiation in the material;   f. introducing an optical delay into the probe beam to compensate for path differences between the probe beam and the pump beam;   g. generating a second, pulsed electromagnetic radiation having a chosen wavelength and directing the second, pulsed electromagnetic radiation into the cell approximately collinear to, but in the opposite direction to the pump beam, whereby the second, pulsed electromagnetic radiation is scattered by the grating formed in the chosen material in the optical cell; and   h. detecting the scattered electromagnetic radiation, whereby an ultrafast time gate is generated in the chosen material in said optical cell which discriminates between the ballistic and snake components, and the diffusive component of the probe beam which exit the turbid medium.   
     
     
       8. The method for imaging structure in a turbid medium as described in claim 7, wherein said step of generating a second, pulsed electromagnetic radiation having a chosen wavelength and directing the second, pulsed electromagnetic radiation into the optical cell approximately collinear to, but in the opposite direction to the pump beam, is achieved by retroreflecting pump beam radiation unabsorbed by the chosen material in the optical cell back into the optical cell, whereby the resulting scattered electromagnetic radiation is the phase conjugate of the probe beam. 
     
     
       9. The method for imaging structure in a turbid medium as described in claim 8, further comprising the step of introducing a chosen delay in the retroreflected unabsorbed electromagnetic radiation such that the delay between the grating formation and the scattering of the retroreflected unabsorbed electromagnetic radiation is optimized. 
     
     
       10. The method for imaging structure in a turbid medium as described in claim 7, wherein the first, pulsed electromagnetic includes frequency-doubled Nd:YAG laser radiation, and the chosen material includes a solution of rhodamine 590. 
     
     
       11. The method for imaging structure in a turbid medium as described in claim 7, further comprising the step of forming the pump beam into a thin sheet which traverses the optical cell substantially along the front surface thereof in order that the probe beam traverse the minimum amount of chosen material before interacting with the pump beam. 
     
     
       12. The method for imaging structure in a turbid medium as described in claim 7, wherein the chosen wavelength of the first, pulsed electromagnetic radiation is selected such that the chosen wavelength is off resonance with the chosen material contained in the optical cell, so that absorption of the pump beam and probe beam in the optical cell are minimized.